The present invention relates generally to quality of speech in a communication system, and in particular to reduction of noise contrast in a speech codec handover.
Many speech codecs provide the ability for speech to be conveyed with toll quality. However, different codec technologies behave differently for the coding of non-speech signals, and in particular, background noise. For example, ADPCM at 32 kb/s (killobits per second), which has developed widespread use as a speech codec for digital cordless and short range systems (such as CT2, DECT, PHS and PACS systems), offers almost transparent quality for any background noise. Liner Prediction Analysis by Synthesis (LPAS) codecs (such as CELP, VSELP, and ACELP systems) which are predominantly used in cellular and digital PMR systems (such as GSM, IS-54, IS-95, iDEN, and TETRA) code background noise with a completely different character.
Mobile systems generally have background noise in transmissions although noise reduction is possible. Mobile systems include the background noise since it is well known that most users prefer some low level of comfort noise to be present to indicate that the call is still active.
Some systems, such as UMTS (Universal Mobile Telephone System), allow handovers between cordless/short range type environments and traditional cellular environments and from cellular to cordless/short range. The UMTS system is designed to allow a user to move about world-wide with a single mobile telephone unit and be able to communicate in any system in that area. The UMTS system also allows links different systems in a single area, such as cordless and cellular. In this type of system, a user may begin a call on their cordless system and then begin moving out of range of that system. In order to maintain the call, the call is handed over from one system to the other, such as from a cordless system to a cellular system. The call may also move from the cellular system to a cordless system as the user moves into the range of the cordless area.
As mentioned above, each speech encoder, or codec, has somewhat differing parameters and design requirements. Because of this, a user will hear different background noises as they use different communication systems incorporating different codecs. Certainly during a handover, where two different systems having differing design parameters are incorporated, the user will be exposed to differing codecs and therefore differing background noise.
In a system where more than one codec is utilized, such as the UMTS system mentioned above, it is desirable for users under handover conditions to be relatively unaware of the changes between background noise characteristics. UMTS aims to provide seamless handovers, and while seamless usually means without break, obvious background noise contrasts will occur during handovers between systems exploiting the different codec technologies.